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Say a beer has been exposed to too much oxygen and it is now giving off cardboard and stale flavors (oxygen-related off-flavors).
Say another beer has been exposed to too much light and is now lightstruck (skunked).
Do any mechanisms exist (at either homebrewer scale or even the commercial/laboratory scale) to reverse either of these flaws?
I think that's sort of like reversing a piece of wood in a fire being burnt. The beer has gone through chemical changes- it's nontrivial to reverse them.
I'm not aware of any commercial or home-scale method that will effectively reverse oxidation. Some people purport that campden tablets can help- but if a beer is well and truly oxidized I don't believe for a moment they'd have a noticeable impact.
Preventing oxidized beer is key- but it's all a preventative game. Purge your containers, don't leave headspace, rack under pressure, etc.
Oxidation is probably the most common and pervasive flaw in the brewing industry (at least in the US) for decades now. If there was a way to undo it, I think it'd be well established.
Cardboard flavours are not the result of oxygenation but of oxidation. Oxygen has chemically bonded to chemical compounds in the beer and permanently changed them. While this process should reversible in theory, it would take an insanely complex laboratory process. Think of it like putting the smoke and the heat back into a used match.
Skunk-ish aromas are the result of hop compounds breaking down (almost exclusively due to exposure to ultraviolet radiation in daylight) and this is, to all intents and purposes, completely irreversible. You cannot identify the breakdown products in the beer, and even if you could there's no way to glue them back together again.
Either toss the batch or run it through a reflux still.
